In electronics, frequency dividers are used to convert a signal at a given frequency to a signal at an (usually even) integer sub-harmonic of that frequency. A specific example is that of divide-by-two circuitry. Each divider has an essentially random state variable that determines the phase of the outputs. Therefore, when two divide-by-two circuits are driven by the same input signal, the outputs of those circuits may be in-phase or anti-phase.
Referring to FIG. 1, an example of a divide-by-two circuit (10) is shown. It consists in this example of two D-type flip-flops (12, 14), both clocked at a common clock node (15). The second flip-flop (14) has an inverse clock input (16). The Qbar output of the second flip-flop 14 is connected to the D input of the first flip-flop 12, and the Q output of the first flip-flop (12) is connected to the D input of the second flip-flop (14). I and Q outputs are derived from the Q output of the first flip-flop (12) and of the second flip-flop (14) respectively. The I and Q outputs provide outputs 90 degrees phase apart, at half the clock rate. Other circuits for divide-by-two operation are known in the art.
Similarly, logic and other circuits for divide-by-n operation are well-known in the art.
Divide-by-two circuitry is used in many applications. A notable one concerns radio transceivers to generate quadrature local oscillator (LO) signals for use in mixers.
In such an application, the phase of the local oscillator signal input to a mixer is transferred to the mixer output; therefore, a randomly occurring 180 degree phase relationship between local oscillator signals leads to a randomly occurring 180 degree phase difference in the transfer function of the transmit or receive chains being driven by those local oscillator signals. Some transceiver operations, such as beam-forming, require that the phase relationships between two transmit or receive paths stays constant over some period. If the dividers are reset or powered down during that period, it is possible for them to start up with the opposite phase relationship than they had previously. This randomly occurring 180 degree phase shift between the transmit or receive chains may be detrimental or even catastrophic to schemes that require knowledge of the phase relationship between those chains.
Additionally, there may be coupling of the divider outputs to the signal paths in the transceiver, leading to local oscillator radiation in the transmitter or DC offset in the receiver. If the phase relationship between the divider outputs is stable over time, calibration may be used to remove the effect of that coupling. However, if the phase relationship between the divider output changes from time to time, such calibration must be performed each time the dividers have an opportunity to change their phase relationship.
One solution would be to keep the dividers enabled over any period in which the phase relationship must stay constant. A shortcoming is that the circuits which generate and distribute the divider input signal, and the dividers themselves, may consume significant power. The inability to disable these blocks to save power during idle periods is a significant cost.
An alternative solution to this issue is to share a divider between the two mixers so as to guarantee a fixed phase relationship for all mixers each time the divider starts up. There are a number of potential issues with this solution. The first is that the quadrature local oscillator signal then needs to be routed from the shared divider to each of the mixers where it is used. This may necessitate long routes to one or more mixers. These long routes may require significant buffering to drive, which may consume a significant amount of power. The second issue is that long routing of quadrature signals may lead to quadrature mismatch. However, this issue is mitigated by any quadrature mismatch calibration that is performed. The third issue is that sharing a divider between several mixers is liable to increase coupling between the mixers, leading to unwanted signal coupling between transmit or receive paths. Separate dividers provide a level of isolation between paths.